1. Field of the Invention
The present invention relates to a multibeam optical scanning device and an image forming apparatus.
2. Description of the Related Art
“An optical scanning device causing a deflecting unit to deflect a light beam from a laser light source and causing a focusing optical system to focus the light beam into a beam spot on a scanning surface to optically scan the scanning surface” is widely known as a device related to such an image forming apparatus as a digital copier, a laser printer, a laser facsimile, and a laser plotter. The optical scanning device offers a variety of variants in its specific applications.
In techniques related to optical scanning, “faster optical scanning” is one of problems to which solutions have been sought. One method for faster image formation by optical scanning is “multibeam optical scanning”, which enables optical writing along a plurality of scanning lines by one stroke of optical scanning, thus significantly improves image formation speed.
Recently, a vertical-cavity surface-emitting semiconductor laser (VCSEL) is put in practical use. In the VCSEL, a plurality of light-emitting sources is arranged easily on the same plane. This makes the VCSEL a preferable light source for the above multibeam optical scanning.
Another problem to be solved with regard to an optical scanning device is that a higher pixel density in optical writing has been in demand to enable highly definite image formation. To form a highly definite image by optical writing, “reducing and stabilizing a beam spot size” is essential.
Stabilizing a beam spot size requires “enlarging the depth allowance of the beam spot”.
The optical scanning device is designed so that the beam waist position of a light beam matches a scanning surface. For this reason, in terms of design, a “beam spot size” is equivalent to a “beam waist diameter” formed on an optical beam focusing unit. To reduce the beam spot size, therefore, the beam waist diameter must be reduced. Meanwhile, the beam diameter of a light beam increases as the light beam goes away from the beam waist position from the “boundary of the beam waist position”.
In actual manufacturing of the optical scanning device, a manufacturing error or assembly error in manufacturing and assembling a component or optical element is inevitably involved in the optical scanning device. In general, therefore, a “gap” due to such an error results between the beam waist position and the “surface of a photosensitive image carrier that is actually the scanning surface”. When the scanning surface position separates from the beam waist position because of this “gap”, the beam spot size on the scanning surface increases regardless of whether the “gap” results at the backward side or forward side in the direction of advance of the light beam. Such an “increase in a beam spot size due to a gap” is called “spot size dilution”, and the above “gap” is called “defocusing”.
A “depth allowance” means an “allowable range of beam spot size dilution” against a design-based beam spot size, that is, a range of defocusing in the optical axis direction in which a “change in a beam spot size is allowable (e.g., 10% or less of a beam waist diameter). If defocusing of a light beam relative to a scanning surface is within the depth allowance, an actual beam spot size on the scanning surface is within the allowable range of spot size dilution, which enables proper optical scanning.
Reducing an error in the manufactured optical scanning device has a limitation, so that the dept allowance should preferably be large as much as possible.
The relational equation d∝w2/λ is satisfied, where “d” is depth allowance, “w” is beam spot size, and “λ” is wavelength. This relational equation means that a “large depth allowance” requires a “large beam spot size”. In other words, a reduction in the beam spot size w and an increase in the depth allowance d run counter to each other. Usually, therefore, reducing the beam spot size w results in the smaller depth allowance d, which degrades the stability of the beam spot size.
For “a reduction in a beam spot size” without the shrinkage of a depth allowance, directly expanding the depth allowance of the beam spot is necessary. A “pseudo-Bessel beam” is known as a “form of light beam” that realizes the expansion of the depth allowance of the beam spot. The pseudo-Bessel beam is generated using an annular thin slit, an axicon prism, or a binary optical element equivalent to the axicon prism (Japanese Patent No. 3507244, Japanese Patent Application Laid-open No. H09-64444, Japanese Patent Application Laid-open No. H04-171415, and Japanese Patent Application Laid-open No. H10-227992).
Another known depth allowance expanding method is a “method of using an optical element that brings light intensity at four corners of an exit pupil higher than light intensity at the center of the pupil” (Japanese Patent Application Laid-open No. 2000-249950).
The Bessel beam or a beam similar to it can be generated by shielding the central part of the beam, but shielding the central part of such a beam having greater intensity results in an extremely large light quantity loss. A method using an axicon prism (or a binary optical element equivalent to the axicon prism) is known as another Bessel beam generating method that avoids light quantity loss.
This method alleviates the above problem of light quantity loss by shielding. According to this method, however, the Bessel beam is generated near the axicon prism, which may pose a limitation in layout work. For example, when the method is applied to the optical scanning device incorporated in the image forming apparatus, a Bessel region and a scanning surface must be in a conjugated arrangement, which requires an additional relay optical system, etc. This may lead to an increase in the number of lenses, a size increase in the optical system, and a cost increase.
At “a beam spot formed by the Bessel beam”, “the light intensity of a side lobe” in a beam profile is extremely high, and the light intensity of high-order side lobe light is also high. For example, when “an ideal (first type in zero order) amplitude distribution following the Bessel function” is obtained as the amplitude distribution of a light beam, the light intensity of first and second lobes is high relative to the light intensity of the main lobe, standing at 16% and 9% of the light intensity of the main lobe, respectively. Because high-order side lobe light occupies a broader area than low-order side lobe light, the light intensity of the main lobe drops widely as the light intensity of high-order side lobe light rises. As a result, since main lobe light is used as signal light, light use efficiency drops substantially.
In optical scanning, too great side lobe light intensity is not preferable because “side lobe light constitutes noise light”. Because high-order side lobe light arises at a location distant from the optical axis, high-order side lobe light is spatially separated from main lobe light having the largest light intensity peak, and tends to act as a noise. In the optical scanning device of the image forming apparatus, when the peak of side lobe light is distant from the peak of the main lobe light, “a thin line of fringe is formed around a dot formed by main lobe light”, which may degrade image quality.
According to the method of Japanese Patent Application Laid-open No. 2000-249950, a beam from a semiconductor laser is “a Gaussian beam having light intensity of the Gaussian distribution”. When this Gaussian beam passes the opening of an aperture, light intensity at “four corners of the opening” on the aperture is lower than light intensity at the center of the opening. To “bring light intensity at four corners of the exit pupil higher than light intensity at the center of the pupil” by converting phase distribution, light at the center have to be shifted to the periphery of the pupil. This encourages the generation of high-order light, and carrying out optical scanning with such a beam spot accompanying high-order light may cause scumming. In avoiding such a trouble, using only the light near the center of the Gaussian beam is effective in suppressing the generation of high-order light. This approach, however, increases an amount of light cut off by shielding to lower light use efficiency.